The inventive concepts disclosed herein are directed to generation of tunable color-balanced light from an electrical energy source.
The landmark invention of the incandescent light bulb by Thomas Edison profoundly impacted society by making it possible for people to operate and function normally after daylight has ended. The color of the light generated by an incandescent light bulb is primarily determined by the black body temperature of the glowing filament, which is generally below about 3400 Kelvin. The electrical to light conversion efficiency of incandescent light bulbs is relatively low—on the order of about 8-10 lumens per watt.
Fluorescent light bulbs generate light by electrically exciting gases in a glass tube to create a plasma that emits high concentrations of ultraviolet light. This ultraviolet light is converted to visible light by a layer of phosphors on the inside of the tube that converts the ultraviolet light to longer visible wavelengths. By selecting the mix of phosphor materials, it is possible to create essentially any output color. Fluorescent light bulbs are much more efficient than incandescent light bulbs, with electrical to light efficiency on the order of about 40-60 lumens per watt. A significant disadvantage of fluorescent light bulbs is that they contain mercury, which is toxic and harmful to the environment.
Light emitting diodes (LEDs) are solid state semiconductor devices that emit light when driven by a bias current. The wavelength of the emitted light is a function of the semiconductor materials used to fabricate the LED device. LEDs are manufactured today with output colors of infrared, red, orange, yellow, green, blue, violet, and ultraviolet. It is possible to create light of any color using a combination of red, green and blue LEDs. The electrical to light conversion efficiency of LEDs is on the order of about 50-90 lumens per watt, which is superior to fluorescent light bulbs without the disadvantages of toxic materials or limited physical characteristics.
One of the challenges for the use of LEDs as individual spectral light sources is cost—creation of white light using sets of three LEDs requires more components and is more complex than fluorescent tubes.
One solution to the cost challenge is the use of a single blue LED arranged to excite and be combined with the output of yellow or red and green phosphors to create white light. A recent improvement in phosphor technology is quantum dots, which are nanoscale particles of semiconductor materials. When excited or “pumped” by relatively short and therefore higher energy light, quantum dots emit light in wavelengths determined by the type of material and the particle diameter. Larger quantum dots with diameter on the order of 10-12 nanometers may emit light in the relatively longer red or orange wavelengths. Smaller quantum dots with diameter on the order of 4-6 nanometers may emit light in the relatively shorter green or blue wavelengths.
An important property of quantum dots is absorption of all wavelengths higher in energy than their bandgap that are then converted into a single color, i.e. they have broad absorption spectra, but narrow emission spectra. This property contributes to improved conversion efficiency from the input pumping light source to the output emission spectra.
One drawback of quantum dot technology is that some of the blue LED pump emission spectrum passes through the conversion layer unconverted, which may adversely affect the balance of the luminance ratio of the constituent wavelengths of the white light. Excessive blue light mixed with yellow or green and red light will result in the white light being bluish in color. Conversely insufficient blue light mixed with yellow or green and red light will result in the white light being yellowish in color.
Another drawback of an LED pumped quantum dot light generation system is failure of the pumping energy to be converted to light through the quantum dot in a single pass. Recovering and reapplying unconverted pumping energy to the quantum dots may improve the conversion efficiency.
One attempt at increasing the conversion efficiency of the pump wavelength has been incorporation of the quantum dots in a diffusing medium thus increasing the effective path length of the light. This diffusing medium does increase overall conversion but also suffers from absorption losses in the backlight cavity.
One important application of white light generating sources is liquid crystal displays (LCDs). LCDs are arrays of very small, electronically switched pixels that alternately transmit or block light through the pixel. Color LCDs employ an array of color filters that control both the amount and color of light that passes through the LCD array. Because the LCD is only an array of light switches or shutters, a light source is needed to provide the light to be transmitted through the LCD. One convenient and efficient geometry for the light source for an LCD is a relatively planar light emitting surface that may be placed in close proximity behind the LCD plane, in a configuration known as direct view backlighting. The quality of the color rendering of a color LCD is dependent on the white color balance of the backlighting source and the color balance of the LCD color filters.